1. Field of the Invention
The present invention relates generally to an electrophotographic (EP) image forming device and, more particularly, to a system and method for adjusting operating parameters, namely, bias voltages of charge and developer rolls, of the image forming device based on selected environmental conditions, namely, wet-bulb temperature values derived from dry-bulb temperature sensor and relative humidity sensor readings, to improve color registration.
2. Description of the Related Art
An EP image forming device, such as a single pass EP printer, typically employs four image forming stations, each one responsible for printing one of four primary colors, typically cyan, magenta, yellow, and black. The individual images, known as separations, produced by each of the four image forming stations are combined to produce the final output image. In tandem print engines, the four image forming stations are aligned in the sheet transport direction such that each separation is formed in succession on the copy sheet as the copy sheet is transported through each print station. Typically, a belt transports the copy sheet. In some printers, a belt serves as an intermediate transfer member (ITM). The image forming stations transfer the individual image separations onto the ITM to form a composite image on the ITM. The composite image is then transferred from the ITM to the copy sheet at a transfer station.
The alignment of the image separations produced by each image forming station is critical to producing a quality printed image. Various factors affect the proper alignment of the image forming stations, such as tolerances, wear, and thermal expansion and contraction. It can be expensive and impractical to control tolerances and wear in order to provide acceptable color registration. Therefore, many printers employ various other techniques to detect and correct for color registration errors.
One technique used to detect color registration errors, referred to as the reverse transfer process, is disclosed in U.S. Pat. No. 7,257,358 assigned to the assignee of the present application. The entire disclosure of this patent is hereby incorporated herein by reference. The basic idea underlying the reverse transfer process, as explained in this patent, is to print a registration mark at a first image forming station and to partially erase or remove the registration mark, printed by the first image forming station, at a second image forming station by reverse transfer of the toner. The registration mark may be printed, for example, on the media transport belt, on an ITM belt, on a media sheet, or some other substrate. The second image forming station does not print a registration mark, but instead partially erases the registration mark printed by the first image forming station to form the final registration pattern. A latent image of a second registration mark is formed by a laser as a discharged area on a photoconductive (PC) drum at the second image forming station, but is not developed. A controller controls the charge of the PC drum and a transfer device so that the PC drum attracts toner from the media transfer belt or ITM belt in areas when the latent image of the second registration mark overlap the first registration mark. When a registration sensor detects that certain specified portions of the first registration mark are completely or nearly completely erased by the overlapping latent image of the second registration mark, there is considered to be no registration error present.
In general, the reverse transfer of toner is maximized when the charge on the PC drum is as high as possible and the voltage of the discharged area is as low as possible. This allows for the largest contrast between charged and discharged areas. The high PC drum charge prevents reverse transfer to areas not discharged by the laser. The low discharge voltage is desirable to create the least negatively charged surface that acts in conjunction with the negative transfer voltage to best attract the negatively-charged toner. Therefore, high charge voltages and high laser energies are desirable.
However, because the developer roll bias in the “reverse transfer” station must be set sufficiently low in order to prevent the development of toner there may be a large difference in potential between the bias on the developer roll and the charge on the PC drum surface. If the potential difference is sufficiently large it may cause Paschen breakdown at the interface between the developer roll and PC drum. (Paschen breakdown voltage is one at which the insulation of air breaks down and an avalanche condition ensues allowing flow of ions.) If there is Paschen breakdown at this interface, some of the toner on the developer roll may become wrong-signed and then transfer to the PC drum and the belt. This wrong-signed toner can interfere with the registration sensor's ability to detect the registration pattern. In some embodiments black toner registration patterns are being sensed on a black belt. Because the reflectivity of the toner may be similar to that of the belt, a higher gain mode may be required. Under these circumstances cyan, magenta, or yellow wrong-signed toner can have a particularly detrimental effect on the sensor's ability to detect the registration patterns.
Paschen breakdown between the developer roll and PC drum also happens at a lower potential difference in certain environmental conditions of temperature and humidity. Although a majority of color laser printers operate in an air-conditioned office environment, such environment may not necessarily be controlled for humidity. It is important that a printer yields high print quality over a wide range of environments. As temperature and humidity of the ambient environment change, the electrical properties of printer components can also change which can have a significant impact on print quality. Heretofore, “cold start” servo voltage has been used to select or adjust charge roll and developer roll biases. Cold start servo voltages are the servo values recorded when the printer is first powered on or after the printer has been idle. However, changes to the printer architecture have made servo algorithms less accurate for optimizing charge roll and developer roll biases to optimize registration operating parameters in all environments.
Thus, when using the reverse transfer process in correcting registration errors, there is a need for an innovation to compensate for environmental conditions of temperature and humidity in order to maintain the maximum potential difference between the charged and discharged areas on the PC drum while still avoiding Paschen breakdowns.